This invention relates to a method of heat-retention in a blind riser and a heat-retaining agent for the blind riser, intending to promote the feeding effect of melt from the blind riser in manufacturing metal castings by using a sand mould.
Generally, in manufacturing castings, to obtain casting products sound in inner quality by preventing the generation of a shrinkage cavity, caused by the solidification shrinkage of molten metal cast into the mould there is provided a riser at or near a thick portion of the product.
Also it is known that in moulds for metal casting there are selectively used, in association with the shape of the casting product and the flask or in relation to the reduction of melt, an open riser in which the upper surface of the riser is open to atmosphere and a blind riser is used in which the upper surface of the riser is closed within the flask.
In the case of a blind riser is used the following means are taken for the purpose of sufficiently exhibiting the effect of the riser thereby to smoothly feed, from the blind riser, the melt equivalent amount of solidification shrinkage amount of the casting:
(1) The blind riser is designed to retain heat in order that the blind riser may not be solidified quicker than the casting. PA0 (2) Atmospheric pressure is introduced within the blind riser to accelerate the transfer of melt from the blind riser to the casting.
As means (1) above, there is employed a method of enclosing the blind riser with a moulded sleeve having an exothermic or heat insulating property, or both, and as means (2) above there is employed an "atmospheric riser" in which a good air-permeating, heat insulating or exothermic core is inserted near the top center of the blind riser. It is also known to use a blind riser sleeve using both the two means (1) and (2) above.
However, these known techniques have the following demerits. That is, in the method of using a moulded sleeve there are mentioned, as demerits, difficulty in mounting the sleeve in the sand mould or pattern, deformation or breakage of the sleeve in the moulding process, and/or release of the sleeve from the sand mould; while with regard to the atmospheric riser the problems are the difficulty of inserting a core into said portion and the sand release caused by the friction at the time of insertion. Further, such demerits either delay the cycle time of moulding or bring about defects in the inner quality of the casting.